(a) Field of the Invention
This invention relates to a method and apparatus for continuously monitoring the electrodeposition of metals and, more particularly, to a method for measuring the activation overpotential of metal deposition and controlling the electrolyte purification and electrodeposition processes in response to deviations of recorded values of the overpotential from the desired values, and an apparatus to carry out the method.
(b) Description of the Prior Art
In processes for electrodeposition of metals such as electrowinning, electro-refining and electro-plating, electrolytes are used which contain impurities which, when present above certain critical concentrations, can electrodeposit with the metal and thereby contaminate or cause re-solution of the deposit with a corresponding decrease in the efficiency of the metal deposition process. To reduce the concentration of impurities in the electrolyte purification procedures may be employed prior to electrolysis. In addition to the purification, one or more polarizing additives may be added to the electrolyte to assist in providing smooth and level deposits, as well as to reduce the effects of remaining impurities.
These polarizing additives act to change polarization. Polarization can be changed by increasing, or decreasing, the concentration of a polarization-causing agent. Polarization also can be changed by decreasing or increasing the concentration of a de-polarization-causing agent. These agents, both polarizing and de-polarizing, can be present in the electrolyte as it comes to the cells from the purification plant, or may be substances, such as animal glue, that are added to the electrolyte to effect control of the electrodeposition process. It is thus to be clearly understood that the term "polarization affecting agents" includes agents of both the polarization-causing and depolarization-causing types. It is also pertinent to note that a given substance will not always act in the same manner in different processes. Thus a substance may act as a polarization agent in one process, as de-polarization agent in another process involving a different metal, or may be inactive in a third process. Similarly it is not unknown for impurities to "catalyse" the effects of polarization affecting agents. Thus it is well known that some experimentation may be needed in order to decide which polarization affecting agents are suitable for any given process.
The procedures presently used for determining the purity of electrolyte are based on chemical analyses and determination of current efficiencies as a measure of impurity content, while those for the addition of suitable polarization affecting agents are based simply on maintaining a constant concentration of agents in the electrolyte despite variations in the quality of the electrolyte. These procedures result in variations in the quality of the deposited metal and the efficiency of the electrodeposition process.
The prior art contains a number of references related to methods for determining the effects of impurities, and addition agents on electrodeposition processes for metals and for determining the purity of electrolyte solutions.
According to U.S. Pat. No. 3,925,168, L. P. Costas, Dec. 9, 1975, there is disclosed a method and apparatus for determining the content of colloidal material, glue or active roughening agent in a copper plating bath by determining the overpotential-current density relationships of solutions having varying known reagent content and comparing the results with that of a solution with a known plating behaviour and roughening agent content. According to Canadian Pat. No. 998,879, C. J. Krauss et al, May 11, 1976, there is disclosed a method for determining and controlling the cathode polarization voltage in relation to current density of a lead refinery electrolyte, wherein the slope of the polarization voltage-current density curve is a measure of the amount of addition agents and wherein the effectiveness of addition agents is changed when the cathode polarization voltage attains values outside the predetermined range of values.
A number of studies are reported in the published literature which relate to similar methods. C. L. Mantell et al (Trans. Met. Soc. of AIME, 236, 718-725, May 1966) determined the feasibility of current-potential curves as an analytical tool for monitoring manganese electrowinning solutions for metallic impurities. Polarization curves related to hydrogen evolution where shown to be sensitive to metallic impurities which affect the cathode surface thereby altering the hydrogen overvoltage. H. S. Jennings et al (Metallurgical Transactions, 4, 921-926, April 1973) describe a method for measuring cathodic polarization curves of copper sulfate solutions containing varying amounts of addition agents by varying an applied voltage and recording the relationship between voltage and current density. O. Vennesland et al (Acta Chem. Scand., 27, 3, 846-850, 1973) studied the effects of antimony, cobalt, and beta-naphthol concentrations in zinc sulfate electrolyte on the current-potential curve by changing the cathode potential at a programmed rate, recording the curves and comparing the curves with a standard. T. N. Anderson et al (Metallurgical Transactions B, 7B, 333-338, September 1976) discuss a method for measuring the concentration of glue in copper refinery electrolyte by determining polarization scan curves, which upon comparison provide a measure of glue concentration. According to U.S. Pat. No. 4,146,437 issued Mar. 27, 1979, T. J. O'Keefe, there is disclosed the use of cyclic voltammetry for the evaluation of zinc and copper sulfate electrolytes. Cyclic voltammograms, which include the cathodic of zinc and copper sulfate current-potential relationships, and polarization curves, are recorded as a means for approximating the quantities of impurities and addition agents in zinc sulfate electrolytes.
This first group of references discloses methods wherein metal is deposited on an electrode and wherein current, or current density-potential, curves represent cathode polarization potentials in relation to varying currents and/or current densities.
T. R. Ingraham et al (Can. Met. Quarterly, 11, 2, 451-454, 1972) describe a meter for measuring the quality of zinc electrolytes for measuring the amount of cathodic hydrogen released during electrodeposition of zinc and indicating current efficiency by comparing the weight of deposited zinc with both the amount of zinc to be expected and the rate of hydrogen evolution. In U.S. Pat. No. 4,013,412, Satoshi Mukae, Mar. 22, 1977, there is disclosed a method for judging purity of purified zinc sulfate solution by subjecting a sample of solution to electrolysis, combusting generated gases and measuring the internal pressure in the combustion chamber which is an indirect measure of current efficiency. M. Maja et al (J. Electrochem. Soc., 118, 9, 1538-1540, 1971) and P. Benvenuti et al (La Metallurgia Italiana, 60, 5, 417-423, 1968) describe methods for detection of impurities and measuring the purity of zinc sulfate solutions by depositing zinc and then dissolving deposited zinc electrolytically and relating calculated current efficiency to impurity content.
This second group of references relates to methods and apparatus for determining electrolyte purity wherein electrolysis of solutions is used to determine current efficiency which is subsequently related to electrolyte purity.
In my co-pending United States Application Ser. No. 052,921, filed June 26th, 1979, there is disclosed a method for controlling a process for the recovery of zinc from a zinc sulfate electrowinning solution which comprises decreasing a potential, which is applied between electrodes in a test cell containing a sample of solution, at a constant rate at substantially zero current, measuring the decreasing potential, terminating the decreasing of the potential at a value corresponding to the point at which zinc starts to deposit, determining the activation overpotential, relating the activation overpotential to the concentration of impurities and adjusting the process to obtain optimum recovery of zinc.
Although the method according to this co-pending application overcomes the necessity for electrolyzing solution to determine current efficiencies or for measuring polarization potentials in relation to varying currents or current densities, several disadvantages still exist. The method is not continuous and it is necessary to determine the value of the activation overpotential for each sample by decreasing the applied potential each time until the value is reached at which zinc starts to deposit and the current density increases from its substantially zero value for a further small decrease in potential.